1. Field of the Invention
The invention relates to the field of ultrasound diagnosis of the cardiovascular systen, particular methods and devices for providing both ultrasound amplitude images and Doppler data from blood flow.
2. Relation to Prior Art
Techniques for ultrasonic blood flow velocity measurement based on the Doppler principle and echo amplitude imaging employing ultrasonic pulses are regarded as prior art, and the invention relates to a combination of these for virtually simultaneous measurements in real time. The method may be implemented by making small modifications in commercial imaging and blood velocity measuring apparati and interconnecting these through a central control unit.
The image of the biological structure is shown on a suitable display screen, and the region in which the blood flow velocity is measured is shown on the same screen. A useful spectrum analysis of the received Doppler signal may also be shown on the display and possibly printed out on a suitable printer.
The object of the invention is to simplify the aiming-in of the ultrasonic head or transducer(s) during Doppler measurement of blood velocity, by utilizing the echo amplitude image for determining the region in which the blood velocity is measured. By combining the blood velocity measurement with a dimensional measurement of blood vessels, it is possible to estimate or calculate the actual volume flow in the vessel.
A very important consideration when using equipment of the type concerned here is that the Doppler signal is presented in audible form, for example in a loudspeaker. The operator carrying out an investigation will to a high degree use the information being presented via the audio Doppler signal, in order to seek regions in the blood system in which more detailed measurement or imaging may be of interest. Therefore, it is very important that the Doppler signal presented audibly is not substantially disturbed or distorted.
Equipment for two-dimensional imaging of biological structures in real time on the basis of the amplitude of echos from a short ultrasonic pulse, is commercially available. Such known equipment is found in two versions:
There is also commercially available equipment which combines echo amplitude imaging and Doppler blood velocity measurements.
Three principles have been used:
The above methods of interrelating amplitude imaging and Doppler measurement have deficiencies and disadvantages as follows:
In methods (b) and (c) a continuous wave Doppler measurement may not be used, which means that the maximum velocities which may be measured are determined by the pulse repetition frequency in the Doppler pulse mode.
These deficiencies are avoided with the present invention by:
The large interrupt interval for amplitude imaging allows for the same scan rate of the ultrasonic beam over the image field as for dedicated imaging. Structures in different parts of the field are then imaged during such a short interval that physiological movement are practically "frozen" during the sweep, and the artifacts due to slow sweep rate for method "c" above are avoided. Also, the Doppler interval can be kept small so that a high frame rate of fully updated images occurs (typ. 20 frames pr sec.). Moreover, both pulsed and continuous wave Doppler measurements can be done during the Doppler interval, and the pulse rate in pulsed mode need not be reduced from what it is for a dedicated Doppler device. Thus the maximum velocity that can be measured is the same as for a freestanding pulsed and continuous wave Doppler in contrast to methods (b) and (c) above.
The substitute signal replaces the directly measured Doppler signal either all of the time or during those portions of the time when the direct signal is not available due to the imaging, and also due to ringing in the tissue filters of the Doppler instrument as discussed below. When the intervals for direct Doppler measurements are much shorter than the intervals when Doppler measurements are interrupted for imaging, it is best to use the substitute signal all of the time. The direct Doppler signal is then used to update the generator for the substitute signal. When the Doppler intervals are comparable or longer than the imaging intervals, and the imaging intervals are not too long (approx. 20 msec.), better overall quality of the audible signal is obtained when substitution is done only when there is no direct Doppler signal available. The quality of the composite signal then generally improves the longer the Doppler interval is, but the frame rate reduces as the length of the Doppler interval increases. Good performance is obtained with an imaging interval of 17 msec and a Doppler interval of 50 msec, giving a total frame time of 67 msec and a frame rate of 15 frames pr sec.
The techniques for obtaining approximate values for signals that are not measured completely are in technical literature called estimation techniques. This concept covers both smoothing of data to remove noise (filtering), estimating values in between measured data values (interpolation), and generation of future data (prediction). The substitute signal described in this patent can be obtained through estimation schemes, but it is important to note that it is not a requirement that the substitute signal be an approximation to the time waveform of the direct Doppler signal; it is only necessary that it has comparable spectral properties and thereby, audible sound, to the signal being gated out. This is sufficient for the use, and gives a simpler estimation scheme than that for estimating the time waveform, since the substitution interval in practical applications is far longer than the correlation time of the Doppler signal. For illustration, a commonly used method for signal waveform prediction is a linear minimum variance predictor. This will give a predicted value equal to the signal mean value (which is zero in this case) for time intervals which are larger than the correlation time of the direct Doppler signal.
There is a fundamental difference between pulse echo amplitude imaging and Doppler measurement of velocity, in that with amplitude imaging, only one pulse is needed in each beam direction to collect the data. For Doppler measurements one must measure over a longer period to estimate the Doppler shift in frequency with sufficient accuracy. Assume that we do Doppler measurement for an interval T. The accuracy in the estimate of the Doppler shift, Df, is then Df=1/T.
Typically we use T &gt;2 msec which gives Df &lt;500 Hz. For amplitude imaging in comparison, the time to collect data for one beam direction is on the order of 200 .mu.sec, i.e. 1/10th of what is needed for Doppler measurements. Thus the imaging portion of the instrument can be rapidly switched on and off without interfering significantly with the measurement, while Doppler measurements must be continuous for a longer period to obtain accuracy.
To this point we have not been fully clear about what we mean by Doppler signal, illuding to tha it is the audio signal obtained during Doppler measurements. Existing Doppler instrumants contain mixercircuits which transfers the received rf-signal either down to base-band, generating two audio quadrature signals to maintain direction of blood flow, or centers the signal around an audio offset frequency. The signal is then fed through filters (high-pass filters for the base-band system, and band-reject filters for the off-set system) to remove strong, low Doppler shift signal components from tissue structers. We call these tissue filters in the following.
One reason to bring the signal down to the audio region is that simpler circuits may be used for analysis, but in principle we might as well do the tissue filtering and further analysis at rf-frequencies. The present invention does not specify at what level the signal processing is done, but is clearly simplest to do it with base-band system.
When switching the Doppler instrument on and off, there will be strong transient ringing in these tissue-filters due to the abrupt excitation at the input of these filters. The Doppler signal will not be useful during this ringing period which lasts on the order of milliseconds. Another way to view this is that some time of measurement is necessary in order to estimate the low frequency components with sufficient accuracy so that they can be removed from the signal.
It is difficult to obtain high quality blood flow Doppler information if one tries to predict or interpolate a missing part of the Doppler signal (due to an imaging interrupt) prior to any tissue filtering, since this signal is comprised of components from both tissue and blood. The Doppler signal from blood is much weaker than the low Doppler shift signal from the tissue structures (typically more than 60 dB weaker), such that even a small fractional error in the estimate will have amplitude comparable to the signal from blood. For example, with a 60 dB ratio, a fractional error of 0.1% in the estimate will give an error comparable to the amplitude of the signal component from blood. This is the reason why the imaging pulses cause strong disturbances in the audio signal when using the method cited under "c" above. In this method, a pulsed wave Doppler measurement is interrupted repetitively for one pulse at a time, to generate a single amplitude imaging pulse and then substitute the lacking Doppler sample with an estimated value, without any prior tissue filtering. As estimate it is suggested to use either the previous Doppler sample or a linear combination of the previous and the following Doppler samples. Both these estimators have so large errors that they give a strong disturbance to the Doppler signal from blood, and it seems difficult to find practical estimators that will not have the same deficiencies. Because of this disturbance, the imaging pulse rate has to be reduced which is responcible for the slow sweeping image update of this method.
The present invention avoids this problem by using the Doppler signal from blood, after the low frequencies from tissue have been removed by tissue filtering, as the basis to generate a substitute signal. By that, the strong echos from tissuse structures have been removed, and the substitute signal has approximately the same amplitude as the Doppler signal from the blood. Therefore a fairly large relative estimation error can be tolerated and still give a small amplitude disturbance relative to the Doppler signal from the blood. Also, since according to the present invention we are only concerned with the spectral properties of the substitute signal, the estimation procedure is more robust than if we were to estimate the time wave form of the Doppler signal. It should also be noted that if we use the signal after the tissue filter as a basis for the estimation, the interruption of the Doppler measurement has to be longer than the transient ringing of the tissue filter, since the ringing disturbes the Doppler signal from the blood. This ringing time is normally larger than the correlation time of the Doppler signal, and it is then difficult to estimate the time waveform of the missing Doppler signal after the tissue filter, as discussed above. Therefore, using a substitution signal as in the present invention, has great advantages.